Power assisted gearboxes, which are structurally similar to a traditional type manual gearbox except for the fact that the clutch pedal and the gear selection lever operable by the user are replaced by corresponding electric or hydraulic servo controls, are increasingly widespread. By using a servo controlled gearbox, the user only needs to send the command to change to a higher gear or to a lower gear to a transmission control unit and the transmission control unit autonomously performs the change of gear by acting both on the engine and on the servo controls associated to the clutch and gearbox.
A command to change gear may be generated manually, i.e. following a command given by the driver, or automatically, i.e. regardless of the operation by the driver. When the command is generated to perform a change of gear, the transmission control unit drives the servo control associated to the clutch to open the clutch so as to mechanically separate a primary shaft of the gearbox from a drive shaft; at the same time, the transmission control unit acts on the engine control unit to temporarily reduce, if required, the driving torque delivered by the engine itself.
Once the transmission control unit has checked the clutch opening, the transmission control unit drives the servo controls associated to the clutch to disengage the currently engaged gear; when the transmission control unit has checked the disengagement of the gear, the transmission control unit drives the servo controls associated to the gear to engage the new gear. Once the transmission control unit has checked that the new gear has been engaged, the transmission control unit drives the servo controls associated to the gear to engage the new gear.
Finally, when the transmission control unit has checked that the new gear has been engaged, the transmission control unit drives the servo control associated to the clutch to close the clutch so as to render the primary shaft of the gearbox and the drive shaft angularly integral with one another; at the same time, the transmission control unit acts on the engine control unit to restore, if required, the driving torque delivered by the engine itself.
It is apparent that as long as the clutch is open, the drive shaft is disconnected from the primary shaft of the gearbox and therefore from the driving wheels; accordingly, as long as the clutch is open, the driving torque generated by the engine is not transmitted to the driving wheels or, in other words, a “torque hole” occurs for the driving wheels. The currently commercially available last generation power assisted gearboxes, which operate in accordance with the above described method, require a time generally in the range from 250 to 600 ms to perform a change of gear; the time actually employed depends both on the dynamic performance of the gearbox components and on the comfort degree required. Accordingly, the torque hole for the driving wheels caused by a change of gear is clearly perceived by the occupants of the vehicle, as it becomes a considerable discontinuity in the longitudinal acceleration of the vehicle.
It must be noted that the torque hole for the driving wheels caused by a change of gear is clearly perceivable by the occupants of the vehicle when the vehicle is accelerating, i.e. during the progression of the gears (change from a lower gear to a higher gear) or during the down shift of the gears during acceleration (change from a higher gear to a lower gear similar to the “kick-down” of a traditional automatic transmission), because during the acceleration of the vehicle the engine is “boosted” to accelerate the vehicle itself. Instead, when the vehicle is slowing down, i.e. during the down shift of the gears during slowing down (change from a higher gear to a lower gear), the torque hole for the driving wheels caused by a gearbox is not normally perceived by the occupants of the vehicle, because the slowing down of the vehicle is substantially performed by the breaking system and the engine does not have an essential role in the dynamics of the vehicle.
Furthermore, during the progression of the gears and the position of the accelerator being the same, the torque hole for the driving wheels caused by a change of gear has a greater effect on the occupants of the vehicle at lower gears (transition from the first gear to the second gear and from the second gear to the third gear), whereas it has a smaller effect on the occupants of the vehicle at higher gears (transition from the third gear to the fourth gear and following) because at lower gears the vehicle has a much higher instant longitudinal acceleration.
Several inquiries carried out among motor vehicle drivers have revealed that the torque hole for the driving wheels caused by a change of gear is accepted when the power assisted gearbox is driven manually, i.e. when the power assisted gearbox is considered as a substitute for a traditional type manual gearbox, in which the torque hole for the driving wheels is normal and inevitable. The torque hole for the driving wheels caused by a change of gear is instead not accepted when the power assisted gearbox is driven automatically, i.e. when the power assisted gearbox is considered as a substitute for an automatic torque converter gearbox, in which the torque hole for the driving wheels is absent. Accordingly, a power assisted gearbox may not be used as a natural substitute for an automatic torque converter gearbox, although the power assisted gearbox displays a considerably higher performance.